Many types of liquid-crystal (LC) device modify the polarization of light travelling through them in a way that is dependent on the root-mean-square (RMS) amplitude of an applied alternating-current (a.c.) electric field. The a.c. electric field is generated by a drive circuit that applies an a.c. drive signal to the electrodes of the cell. The magnitude of the polarization change is a continuous function of the RMS value of the drive signal. The RMS value of the drive signal is in turn defined by an input value received by the drive circuit.
In conjunction with polarization-selective optical components, LC devices can be used to build useful devices such as displays, optical switches, optical multiplexers and electrically-controllable optical attenuators. Many applications, notably those related to optical communication networks, require the drive circuit to provide a fine control over the electrical drive conditions of the LC device, as well as long-term stability.
Another desirable property of drive circuits for LC devices is that they generate a drive signal that is a pure a.c. signal with little, and preferably no, DC component. Most LC devices are damaged by the long-term application of even a small DC voltage across them.
Analog drive circuits that generate an a.c. drive signal whose RMS value is determined by an analog sample received by the drive circuit are known in the art. An example of such an analog drive circuit for an LC device is described in U.S. Pat. No. 5,977,940 to Akiyama et al. However, in an increasing number of applications, a digital input value is provided as the input signal for the drive circuit. To operate with a digital input value, the conventional analog drive circuit needs to be preceded by a digital-to-analog converter. This substantially increases the complexity of the device incorporating the analog drive circuit.
Thus, what is needed is a simple drive circuit that can generate an a.c. drive signal whose amplitude is defined by a digital input value. What is also needed is a drive circuit that can generate an a.c. drive signal suitable for driving an LC device.
What is also needed for driving LC devices used in display applications is a drive circuit that can generate multiple drive signals, each in response to a respective digital input value, and that is not significantly more complex than a drive circuit that generates a single drive signal.
What is also needed is a drive circuit capable of generating an a.c. drive signal that additionally includes a baseline a.c. component whose amplitude is defined independently of the digital input value. Such drive circuit enables the apparent brightness of all the LC devices constituting part of a display to be set independently of the digital input value that defines the brightness of each individual LC device, for example.
What is also needed is a drive circuit in which a P-bit digital input value defines the amplitude of the pure a.c. drive signal with a precision of one part in 2B, where P<B.
What is also needed is a drive circuit capable of generating an a.c. drive signal that includes a DC component having a level defined independently of the digital input value.
Drive circuits that can generate an a.c. drive signal whose RMS value is defined by a digital input value, and that may additionally include either or both a baseline a.c. component whose RMS value is defined independently of the digital input value and a DC component whose level is defined independently of the digital input value are needed for driving LC devices and for other applications.